1. Field of the Invention
The present invention relates generally to a thermal transfer printer of the kind employing a ribbon carrying ink or pigment and, more particularly, to a system for driving the thermal head to print onto a record medium.
2. Description of the Prior Art
Thermal transfer printing apparatus is known that uses an inked or pigment-carrying ribbon to print a line of information or data at high speeds. These "line" printers have many applications apart from data processing applications, for example, a thermal head having 512 heat elements arranged in line can be used to copy a television image from a video signal by vertically scanning the video frame and using the thermal head to print the image onto a movable paper sheet. In that application, the thermal energy or heat needed to produce the image requires high electrical power levels in relation to the power requirements of the video processing circuitry and to the input power that is available. In order to meet these high power requirements it is known to arrange the heat elements of the thermal head so that they are electrically divided into a number of groups. Typically, in such thermal head arrangement, the groups of heat elements are sequentially driven in a time-sharing manner with respect to the power source, so that all heat elements are not energized simultaneously.
In the printing apparatus described above, the heat elements comprising the thermal head are divided into upper and lower groups (or first and second groups) relative to the longitudinal direction of the thermal head. The upper or first group of heat elements is driven or energized during a first heating cycle, and the lower or second group of heat elements is driven or energized during the next or second heating cycle. Depending on the length of the line to be printed and the particular power requirements, the thermal head can be divided into more than two groups of heat elements. The operation of the heat elements in the vicinity of the boundary between the groups presents a problem, because immediately after the upper group of heat elements is operated, "residual heat" is conducted to the lower group of heat elements, so that some of the heat elements in this second or lower group adjacent the upper group are preheated and/or additionally heated by heat from the upper groups. Thus, when the lower group is energized, various of the heat elements will get hotter than they would if the heat were due solely to the energization power. As a result, a stripe, or "overdeveloped" area is noticeable at the center of the printed image or at each interface between the groups of heating elements forming the head, thereby resulting in poor appearance and unacceptable print quality.